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2.
Guys riding 2-up on a motorcycle; or more generally, riding any 2-person device where one individual sits directly behind the other (second person being a male).
I wanted a ride on his motorcycle, but was uncomfortable going nut to butt.
by keeena January 07, 2008
 
1.
a situation where a group of people usually males are packed into a small space and end up extremely close together
Everyone in this elevator is nut to butt.
by heywouldjablowme April 28, 2004